1. Field of the Invention
The present invention relates to low gel strength agar-agar or agar which has a low gel strength and does not coagulate at normal temperatures.
2. Description of the Related Art
Gelling agents include gelatin, starch, carrageenan, and agar. Of these, the agar, which can produce stickiness-free gel with crispy texture, is made from red algae including Gelidium and Gracilaria. The agar has special properties, unlike other gelling agents, and is applicable to various uses.
One of parameters showing the properties of agar is gel strength which indicates the solidity of gel. In general, the gel strength is proportional to the agar concentration in a solution. Thus, if agar with high gel strength is used, a gel with desired solidity can be obtained with a small amount of such agar. Under the circumstances, there is a trend in which agar with high gel strength is developed.
The gel strength of normal agar is 400 g/cm.sup.2 or above in the case of powdered agar or flaked agar manufactured by industrial processes, and is 250 to 400 g/cm.sup.2 in the case of square agar or strip agar.
The gel strength is measured in the following manner. A solution containing 1.5% of agar, which is to be measured, is prepared. The solution is left for 15 hours at 20.degree. C. and coagulated. A load is applied on the surface of the thus obtained gel, and a maximum weight of the load, which the gel can withstand for 20 seconds, is measured. The value of the maximum weight per 1 cm.sup.2 is called the gel strength of agar.
In contrast to the above-mentioned gel strength agar, there is a case where low gel strength of about 100 g/cm.sup.2 is required. For example, such low gel strength is needed in agar used as a gelling agent for spread foods, cosmetics and soft-texture confectionery, or used as a fat replacer. In this case, in order to use normal agar, it is necessary to decrease the agar concentration, thereby lowering the gel strength. If the agar concentration is lowered to a certain level, however, water removal occurs and elasticity of texture is lost. In order to prevent such problems from arising, low gel strength agar is required, which has a low gel strength even if the agar concentration exceeds a certain level, and can produce a soft gel with no syneresis.
However, there is no conventional agar which can achieve a low gel strength of about 100 g/cm.sup.2 when the agar concentration is 1.5%. One reason for this is that molecules of agar components (agarose and agaropectin) are long. Specifically, in a solution-state agar, molecules are present in a random coil state, as shown in FIG. 6A. When the solution-state agar is cooled, a three-dimensional network of a double helix structure is formed, as shown in FIG. 6B. Then, the sol state of the agar changes to the association state, as shown in FIG. 6C. Because of this gelation mechanism, the gel strength cannot be lowered if the molecules of agar are long.
Another reason for the high gel strength of conventional agar lies in the manufacturing process. In the prior art, seaweeds such as Gelidium and Gracilaria are used as raw material, and an agar solution is extracted. The agar solution is filtered and then cooled. Thus, the solution is once gelatinized. The obtained gel is pressurized and dehydrated, or the gel is frozen and thawed to separate a water component and then it is dehydrated. Thus, dried agar is obtained.
In the case of employing the pressurizing/thawing process, when an agar gel of a low gel strength is interposed between dehydrating cloths and pressurized, clogging occurs in the cloths and dehydration is not performed desirably. On the other hand, in the case of employing the freezing/denaturing process, an agar gel of a low gel strength does not have an orderly spongy structure, and it is caused to flow out with water. For these reasons, in the prior art, only agar with a predetermined gel strength or above can be obtained.